In the process of electrophotographic printing, the photoconductive members are uniformly charged and exposed to a light image of an original document. Exposure of the photoconductive member records an electrostatic latent image corresponding to the informational areas contained within the original document. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing a developer material into contact therewith. Generally, the developer material comprises toner particles adhering triboelectrically to carrier granules. The toner particles are attracted from the carrier granules to form a toner powder image on the photoconductive member which corresponds to the informational areas contained within the original document. This toner powder image is substantially transferred to a copy sheet and permanently affixed thereto in image configuration.
Electrophotographic copiers and printers utilize distinct voltage levels on the photoconductor surface V.sub.0 levels for each color separation, requiring dynamic frame-to-frame changes in V.sub.0. Even mono-color machines may have different operating modes, e.g., text and photo, requiring substantial frame-to-frame changes in V.sub.0. Furthermore, the desired V.sub.0 levels may depend on the measured relative humidity (RH), to compensate for the developer sensitivity to RH and may change over time as the photoconductor voltage changes.
For grid-control corona chargers, the grid voltage, V.sub.grid, is set to obtain the desired potential, V.sub.0, on the photoconductor surface. Unfortunately, the relationship between grid voltage and V.sub.0 is affected by the variable emission of the corona wires and the variable charge acceptance of the photoconductor. Other factors such as aging, wear, contamination, corrosion, and variable environmental conditions can all add variability to the nominal V.sub.grid -to-V.sub.0 relationship.
In the prior art, a fixed relationship or calibration is assumed between V.sub.grid and V.sub.0. Accuracy and repeatability of V.sub.0 are degraded from the ideal, due to the variability described above, leading to inconsistent and unsatisfactory image quality.
Previous approaches deal with charging variability having utilized feedback control with an on-line electrometer. V.sub.0 is measured, and V.sub.grid is adjusted on a continuous or sampled basis to maintain the desired V.sub.0.
U.S. Pat. No. 4,697,920 in the name of Palm et al discloses a print quality monitoring system with an electrometer to monitor test patch areas on the photoconductor. Some patches are charged and unexposed; others are charged and exposed through color separation filters to images of cyan, magenta, and yellow reference bars. The primary corona charge input voltage is adjusted until the electrometer measurements are within acceptable ranges. The charger adjustment depends on the error from the desired photoconductor voltage level according to a look-up table (LUT). This look-up table is not updated, and yields adjustments rather than absolute charger settings, to be applied to the charger in an iterative procedure.
U.S. Pat. No. 4,796,064 in the name of Torrey discloses charger adjustment with both a "predictive" and an "adaptive" component. The predictive component is based on the rest/run history of the photoconductor. The adaptive component is accomplished by an iterative measure-and-adjust cycle, until the one desired photoconductor voltage is obtained. It is asserted that the same adjustment applies also when there is a different desired photoconductive voltage, but for the most accurate adjustment, the iterative process would have to be repeated.
U.S. Pat. No. 4,512,652 in the name of Buck et al adjusts the charging current in a predetermined open-loop manner as a function of rest time between successive copy cycles to attain a specific target surface voltage. One drawback with this arrangement is that there is no provision for a range of target voltages or updating the adjustment function based on surface voltage measurements fed back.
U.S. Pat. No. 4,348,099 in the name of Fantozzi is directed to a multi-loop feedback control system for a reproduction machine. One of the feedback loops comprises an automatically adjusted corona charging device with a feedback control loop to regulate the dark development potential to a desired value despite the effects of fatigue and age. Once again, there is no provision for switching immediately through a range of target surface voltages. Any changes in the target voltage would require time for the close-loop to converge to the new target, owing to the delay between the time the adjustment is applied and the time the resultant surface potential can be measured.
U.S. Pat. No. 4,355,885 in the name of Nagashima relates to a feedback control loop around the corona charging unit, wherein the charger adjustments are reduced in successive iterations of the measure-calculate-adjust procedure, to improve the speed and accuracy of the conversions of the desired surface voltage. Again, there is no provision for switching immediately through a range of target surface voltages.
U.S. Pat. No. 4,484,811 in the name of Nakahata discloses methods for use in inspection and service to check the desired surface voltages attained when an exposure device is adjusted through a range. There is no provision for the updatable look-up table to be used in the normal imaging mode, relating the corona adjustment to a range of target dark surface voltages.
Thus, it can be seen that it would be inconvenient and time consuming to use feedback control to converge the desired V.sub.0 every time the desired V.sub.0 changes. This is owing to the displacement or transport delay between the corona charger and the measuring electrometer, which determines the minimum time to accomplish a single iteration in the feedback adjustment procedure. Instead, a calibration cycle is run from time to time to determine the V.sub.grid to V.sub.0 relationship over a wide range of voltage. The resultant calibration table is used to immediately set the appropriate V.sub.grid whenever the desired V.sub.0 is changed, interpolating from the table entries, if necessary. There is no need for a time consuming feedback control cycle to converge to the desired V.sub.0 level each time the desired V.sub.0 changes.